1. Field of the Invention
The present invention relates generally to optical fiber couplers, and particularly to lossless coupling in a closed-loop to form a high-Q optical resonator.
2. Technical Background
In general, index-guiding waveguiding structures are known, such as standard optical fibers and planar waveguides operating by themselves as couplers or other light-guiding structures for confining and directing electromagnetic waves in a direction determined by its physical boundaries. Total Internal Reflection (TIR) is the known index-guiding mechanism for propagating the optical mode along the standard fiber axis. Low-loss waveguides result from confinement of the optical energy to the center of the waveguide using such index guiding. With TIR, the optical mode of the standard fiber, will not exist substantially circumferentially outside of the required outer cladding layer that has a lower index of refraction than the higher-index waveguiding core as an evanescent wave.
Evanescent fiber sensors and couplers based on standard fibers are known in the form of “D” shaped fibers. The preform from which a “D” fiber is drawn is polished away on one side until the core is close to the surface of the fiber. The fiber is then drawn and the thin layer of cladding glass remaining adjacent to the core in the previously polished region is etched away over a short length of fiber. The evanescent field of light propagating in the fiber is thus readily accessible only over that short length on a side of the fiber, not substantially circumferentially outside the entire tangential surface of the fiber. Evanescent fiber sensors and couplers can also be fabricated by redrawing an optical fiber so that the optical mode exists partially outside of the cladding layer. Similar to the “D” fiber geometry, the optical mode still propagates down the fiber axis.
The fiber itself can be used as an energy storage element as one type of a closed-loop or ring resonator. Two fiber directional couplers can be coupled to a long length of fiber laterally configured into a circular ring to form an optical all-fiber ring resonator or oscillator that has a high optical path length and a high free spectral range (FSR). Oscillating or resonating signals may then be generated around the ring being the energy storage element. The quality factor Q, or the energy storage time, of the energy storage element determines the spectral linewidth of the respective oscillating signal which can be used for a lot of different applications.
For much smaller devices, with high Q's, whispering-gallery mode resonators are used as another type of closed-loop resonators. Whispering-gallery mode or optical micro-cavity resonators or oscillators have been implemented by planar waveguides or microspheres coupled to etched, processed, or other non-uniformly smooth fibers or various combination of these components.
The high resonances encountered in these microcavities are due to whispering-gallery-modes (WGM) that are supported within the microcavities. As a result of their small size and high cavity Q, interest has recently grown in potential applications of microcavities to fields such as electro-optics, microlaser development, measurement science, high-precision spectroscopy, signal processing, sensing, modulating, switching, multiplexing, and filtering. By making use of these high Q values, microspheric cavities have the potential to provide unprecedented performance in numerous applications. For example, these microspheric cavities may be useful in applications that call for ultra-narrow linewidths, long energy decay times, large energy densities, and fine sensing of environmental changes, to cite just a few examples. In particular, a significant potential application for microcavity resonator devices is adaptation into known chemical/biological agent sensing devices. Chemical sensors known in the art include MEMS (microelectromechanical systems) chemical sensors, optical waveguide-based sensors, surface plasmon resonance (SPR) chemical sensors, surface acoustic wave (SAW) chemical sensors, mass spectrometers, and IR (infrared) absorption spectrometers. Miniaturized sensors, such as prior art MEMS sensors, provide significant advantages. For example, they would be well adapted for in situ functioning. Also, they would be small enough to be deployed in large numbers and implemented for remote probing.
High-Q resonators require that the optical path around the resonator loop be low loss. Therefore it is important that these resonators provide optical guiding in both lateral and transverse directions in order to minimize optical loss (lateral direction is perpendicular to the propagation direction while transverse direction is perpendicular to the direction of propagation and also perpendicular to the plane of the waveguide). Most conventional ring resonator configurations, such as planar ring resonators, spherical resonators, and spliced fiber ring resonators, use some guiding mechanism to make sure that the guided mode does not spread laterally (in a direction perpendicular to the plane of curvature of the resonator).
However, many difficulties present themselves when conventional planar or fiber processed technology, i.e. etching, is used in order to fabricate high quality optical resonators, because the planar or fiber surfaces must show deviations of less than about a few nanometers to minimize scattering optical loss due to the inhomogeneity or other irregularities on the surface. Optical microsphere resonators, on the other hand, can have Q's that are several orders of magnitude better than typical surface etched optical micro-resonators, because these microcavities can be shaped by natural surface tension forces during a liquid state fabrication, such as in the well-known fiber-drawing process. These microcavities are inexpensive, simple to fabricate, and are compatible with integrated optics.
Coupling efficiency is highly dependent on how the ring resonator is used. The efficiency is affected by factors such as: the planar waveguide geometry, the distance between the cylinder, ring, or sphere and planar waveguide, the interaction length, the coupling index. The efficiency is thus highly application specific and complicated to maximize.
Thus, even with microsphere resonators, in order for the potential of microcavity-based devices to be realized, it is necessary to couple light selectively and efficiently into the microspheres. Since the ultra-high Q values of microcavities are the result of energy that is tightly bound inside the cavity, optical energy must be coupled in and out of the high Q cavities, without negatively affecting the Q. Further, the stable integration of the microcavities with the input and output light coupling media should be achieved. Also, controlling the excitation of resonant modes within these microcavities is necessary for proper device performance, but presents a challenge for conventional waveguides.
Typically, good overall performance is gained by accessing the evanescent field in a waveguide. Also, only waveguide structures provide easy alignment and discrete, clearly defined ports. However, power extraction from the input optical radiation has proved to be inefficient for conventional planar waveguides due to cavity and waveguide mode leakage into the substrate and into the modes within the fiber cladding.
It is already known that passive alignment of a cylinder resonator to a planar waveguide is desirable when evanescent optical coupling occurs. However prior structures are not optimum for coupling or alignment with only the relative transverse positioning maintained. More important than the transverse position is the relative vertical position of the waveguide and resonator. These prior structures do not provide this alignment or are overly complex.
In known ring resonator approaches where a planar waveguide is combined with a circular structure for confining whispering-gallery modes, the resonator guiding structure is optimized for coupling to guided modes with relatively small lateral (parallel to the plane of a planar waveguide) and transverse (perpendicular to the plane of a planar waveguide) extents (e.g. 5-20 um in width) or guidance of the resonator. Extent is the mode field width in either the lateral (parallel to the substrate plane or fiber axis) or transverse (perpendicular to the substrate plane or fiber axis) direction. For optical wavelengths in the 0.5-2.0 um range, this mode field will diverge rapidly if no mode guiding mechanism is provided.
In the case of a spherical resonator coupled to a planar waveguide or a tapered fiber guided mode, such as in U.S. Pat. No. 6,583,399, radial mode confinement is provided by the high index difference between the surface of the sphere and the air cladding, combined with a natural outward shifting of the mode due to its constantly bending path in propagation of the light around the sphere. Azimuthal confinement (parallel to the plane of the planar waveguide) is naturally provided by the curved surface of the sphere, which produces the equivalent of a graded-index profile in the azimuthal direction.
In the case of a cylindrical resonator coupled to a planar waveguide or tapered fiber guided mode such as in U.S. patent application 2002/0081055 and U.S. patent application 2002/0044739, radial mode confinement is again provided by the high index difference between the surface of the sphere and the air cladding, combined with a natural outward shifting of the mode due to its constantly bending path in propagation around the sphere. Lateral confinement (parallel to the plane of a planar waveguide) is provided by local removal, deposition, or alteration of guiding material immediately adjacent to the resonator waveguide. However, such processing methods to make the cylindrical resonator are hard to control with the potential of varying scattering losses.
It is therefore desirable to overcome the current problems by providing small, high-Q optical resonators that are precision controllable for maximum mode guidance, manufacturable, and cost-effective, for various applications including biological or chemical sensors with improved resolution.